Completion of boreholes by casing and cementing is well known. Following drilling of the borehole, a tubular casing, typically formed from steel tubes in an end to end string is placed in the borehole and cement is pumped through the casing and into the annulus formed between the casing and the borehole wall. Once set, the cemented casing provides physical support for the borehole and prevents fluid communication between the various formations or from the formations to the surface (zonal isolation). However, problems can occur if drilling mud remains in the borehole when the cement is placed, or microannuli form around the casing and/or borehole wall. The effect of these can be to provide fluid communication paths between the various formations or back to the surface and consequent loss of zonal isolation.
There are various well-known problems associated with conventional cementing operations. For example, drilling must be interrupted and the drill string withdrawn from the borehole each time a casing is to be set; and each casing reduces the diameter of the well.
WO 9706346 A (DRILLFLEX) 20 Feb. 1997 describes a technique in which a tubular preform is introduced into a well on an electric cable and expanded into contact with the wall of the well by inflation of a sleeve located inside the preform. Once inflated, the preform is solidified by polymerisation, typically by heating by means of an embedded heating wire, or by introduction of a heated liquid into the sleeve. Such a technique is typically used for repair of a casing or tubing that is already installed in the well, or to shut off perforations that are producing unwanted fluid such as water (see, for example, the PatchFlex service of Schlumberger/Drillflex).
This invention aims to address some of the known problems with borehole lining by providing a technique that can reduce the interruption to drilling and decrease in borehole diameter.